Laundry appliance

ABSTRACT

A balancing system for a laundry appliance is disclosed, which is particularly suited to horizontal axis washing machines. The system includes force and acceleration transducers at each end of the drum, to detect the imbalance. Several algorithms are disclosed, which calculate the correction required to correct for the imbalance. The correction is implemented using a set of balancing chambers, at each end of the drum. The advantages include the traditional mechanical suspension to be dispensed with, and that the system is capable of adapting to different types of floors.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to a system for balancing the load in a laundryappliance, particularly but not solely, a system for balancing the loadin a horizontal axis washing machine.

2. Description of the Prior Art

Conventional horizontal axis washing machines involve a final spin cycleto extract the washed articles of as much of water as possible toreducing drying tie. However, the requirement of a high spin speed is atodds with quiet operation At the beginning of a spin the cycle the washload can be quite severely unbalanced, such that when the machine triesto accelerate noise and stressful vibrations result.

The means that washing machine designers have employed so far to caterfor imbalance in the load, is typically to suspend the internal assemblyon springs and dampers in order to isolate its vibration, The difficultyis these suspension assemblies never isolate the vibration completely,and as the machine ages they deteriorate and the problem gets worse.Also, these suspension assemblies require significant internalclearance, and so valuable load capacity is lost when designing amachine to standard outside dimensions. Further, because the internalassembly must still withstand the forces due to the imbalance,considerable extra costs result.

The ideal approach is to eliminate the problem at its source, for whichthere are various solutions. The first possibility is to ensure that thewash load is evenly distributed prior to spinning. This is an effectivesolution but it is extremely difficult to achieve in practice. Thereforewhile steps can be taken to reduce the degree of imbalance that must becatered for, it is not possible to eliminate it sufficiently to ignoreit there after. Another approach is to determine the size and nature ofthe imbalance, and add an imbalance that exactly counteracts the first.

Methods of compensating for imbalance in horizontal axis washingmachines have been disclosed in U.S. Pat. No. 5,280,660 (Pellerin etal.), European Patent 856604 (Fagor, S. Coop). These disclosures relateto the use of three axially orientated chambers running the length ofthe drum, displaced evenly around the periphery of the drum, which whenindividually filled with water in the appropriate amounts can be used toapproximately correct imbalances in the axis of rotation.

The disadvantage to these systems is that the imbalance may not becentered along the axis of rotation, and since no control is availablealong the axis of rotation this form of balancing will only ever bepartially successful. This may mean that a suspension system may stillbe required to isolate the vibrations, which adds cost and may reducethe useful life of the appliance.

Static Imbalance

When an object of some shape or form is spun about a particular axis,there are two types of imbalance that it may exhibit: Static andDynamic. Static imbalance is where axis of rotation does not passthrough the Centre of Gravity (CoG) of the object. This means that aforce, F, must be applied to the object (acting through the CoG) to keepaccelerating the object towards the axis of rotation. This force mustcome from the surrounding structure and of course its direction rotateswith the object, as illustrated in FIG. 1. There are two pieces ofinformation required to define a static imbalance 3. They are themagnitude of the imbalance 1 (the moment of the CoG about the spin axis,which in SI units has dimensions kg m), and some angle 2 between thedirection of the offset of the CoG and some reference direction withinthe object 4.

When mounted on a horizontal rotation axis, and under the influence ofgravity, an object with a static imbalance will rotate until its CoGlies vertically under its axis of rotation. This also has theconsequence that a horizontal axis machine, running at speeds slowerthan its resonance on its suspension and at constant power input, willexhibit a slight fluctuation in rotation speed as the CoG goes up oneside and down the other. Unfortunately this is not a feasible techniquefor determining static imbalance at anything other than very slowspeeds.

Dynamic Imbalance

Dynamic Imbalance is a little more complicated. In FIG. 2 the axis ofrotation 5 is not parallel with one of the principle axes 6 of theobject. The principal axes of an object are the axes about which theobject will naturally spin.

For example, consider a short length of uniform cylinder 7 set to spinabout its axis of extrusion, and thus is both statically and dynamicallybalanced Two weights are now attached to the inside of the cylinder, one8 at one end and the other 9 at the other end but on the opposite sidefrom the first one. The CoG 10 of the object has not been moved and soit is still statically balanced, but now spinning the cylinder willcause vibration; it has a dynamic imbalance. Static imbalance can bedetected statically by seeing which way up the object rolls over torest. Dynamic imbalance can only be detected with the cylinder spinning,i.e. dynamically.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a balancing systemfor a laundry appliance which goes as far as is practical for itspurpose towards overcoming the above mentioned disadvantages.

Accordingly in a first aspect, the present invention consists in alaundry appliance comprising:

a perforated rotatable drum for dehydrating a clothes load,

a substantially rigid, free standing drum support means supporting saiddrum rotatably but non-translatably in relation to a support surface,

driving means for rotating said drum at speed thereby dehydrating theload, and

a system for compensating for imbalances of said drum and any loadcarried therein during dehydration of the load.

In a second aspect, the present invention consists in a laundryappliance having a perforated drum for dehydrating a clothes load,driving means adapted to rotate said drum at speed thereby dehydratingthe load and a system for compensating for imbalances of said drum andany load carried therein during dehydration of the load, said systemcomprising:

first sensing means located at more than one position on the drum spinaxis for detecting dynamic rotational imbalance in the load,

a digital processor which in use receives as inputs signals from saidsensing means, and programmed to calculate the value and position of oneor more masses required to be added to the drum to correct the sensedimbalance,

correction means for adding two or more masses to said drum, wherein inuse at least one of said masses being axially spaced from the remainderof said masses and said processor controlling such additions such thatthe resultant value and position is substantially similar as thecalculated value and position to correct the imbalance.

In a third aspect, the present invention consists in a laundry appliancehaving a perforated drum for dehydrating a clothes load, driving meansadapted to rotate said drum at speed thereby dehydrating the load and asystem for compensating for imbalances of said drum and any load carriedtherein during dehydration of the load, said system comprising:

first sensing means located at more than one position on the spin axisof said drum for detecting rotational imbalance in the load,

correction means for adding two or more masses to said drum to correctfor any imbalance caused by the rotation thereof, and

a digital processor which in use receives as inputs signals from saidsensing means and programmed with software causing said processor tocarry out the following steps.

a) energising said driving means to apply a first predetermined rate ofrotation to said drum;

b) instructing said correction means to add at least one small imbalanceto at least one end of said drum and storing the detected rotationalimbalances at each end of said drum;

c) determining the differential relationship between said at least oneadded imbalances' and said detected rotational imbalances' at each endof said drum, thereby estimating the value and position of one or moremasses required to be added to the drum to correct the actual imbalance;and

d) controlling additions of one or more masses to said drum by saidcorrection means such that the resultant value and position of the addedmasses is substantially similar to the said estimated value and positionto correct the imbalance.

In a fourth aspect, the present invention consists in a laundryappliance having a perforated drum for dehydrating a clothes load,driving means adapted to rotate said drum at speed thereby dehydratingthe load and a system for compensating for imbalances of said drum andany load carried therein during dehydration of the load, said systemcomprising:

first sensing means located at one or more positions on the spin axis ofsaid drum for detecting rotational imbalance in the load with respect tothe spin axis of said drum,

second sensing means located at one or more positions on the spin axisof said drum for determining the absolute acceleration of the spin axisof said drum,

a digital processor which in use receives as inputs signals from saidfirst and second sensing means and programmed to estimate the value andposition of one or more masses required to be added to the drum tocorrect the sensed imbalance,

correction means for adding one or more masses to said drum, saidprocessor in use controlling such additions such that the resultantvalue and position of the added masses is substantially similar as thesaid estimated value and position to correct the imbalance.

The invention consists in the foregoing and also envisages constructionsof which the following gives examples,

BRIEF DESCRIPTION OF THE DRAWINGS

One preferred form of the present invention will now be described withreference to the accompanying drawings in which;

FIG. 1 is an illustration of the concept of static imbalance,

FIG. 2 is an illustration of the concept of dynamic imbalance,

FIG. 3 is a cutaway perspective view of a washing machine according tothe present invention with the cutaway to show the machine substantiallyin cross section,

FIG. 4 is an assembly drawing in perspective view of the washing machineof FIG. 3 showing the various major parts that go together to form themachine,

FIG. 5 is an illustration of the drum bearing mount,

FIG. 6 is an illustration of the drum, showing the balancing chambersand sensors,

FIG. 7 is a diagrammatic representation of the liquid supply andelectrical systems of the washing machine of FIG. 3,

FIG. 8 is a waveform diagram giving example output waveforms from thevibration sensors,

FIG. 9 is a graph illustrating the weighting curves,

FIG. 10 is an illustration of the decision making process regardingfilling of the balancing chambers,

FIG. 11 is a flow diagram showing the Imbalance Detection Algorithm,

FIG. 12 is a flow diagram showing the Balance Correction Algorithm,

FIG. 13 is a flow diagram showing the Spin Algorithm, and

FIG. 14 is a block diagram of the equivalent spring system when thelaundry appliance is supported on a flexible floor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a novel method of balancing the load in alaundry appliance, particularly suited to washing machines. Such asystem dispenses with the need for suspension, and this significantlysimplifies the machine design. The following description is withreference to a horizontal axis machine. However it will be appreciatedthat the present invention will be applicable to off horizontal andvertical machines, as well as rotating laundry appliances in general.

General Appliance Construction

The present invention will be described primarily with reference to alaundry washing machine although many of the principles are equallyapplicable to laundry drying machines. FIGS. 3 and 4. show a washingmachine of the horizontal axis type, having a perforated drum 11supported with its axis substantially horizontal in side-to-sideorientation within a cabinet 12. The cabinet 12 includes surfaces whichconfine wash or rinse liquid leaving the drum within a water tightenclosure. Some parts of the cabinet structure 12 may be formed togetherwith the liquid confining surfaces by for example twin-sheetthermoforming. In particular the back and side walls of the machine maybe formed in this way.

The laundry handling system including the drum and many other componentsis preferably contained in a top loading configuration. In FIG. 3 thehorizontal axis spin drum 11 is contained within a substantiallyrectangular cabinet 12 with access being provided via a hinged lid 14 onthe top of the machine. Other horizontal axis configurations may beadopted.

The drum 11 is rotatably supported by bearings 15 at either end which inturn are each supported by a drum support 16. In the embodiment depictedthe bearings are axially located, externally, on a shaft means 19protruding from the hub area 20 of the drum ends 21,22. Other axialconfigurations are equally possible, for example internally located in awell in the outer face of the hub area of the drum to be located on ashaft protruding from the drum support. The drum supports 16 are showneach as a base supported unit and have integrated form, which again isideally suited to manufacture by twin sheet thermoforming, blow mouldingor the like. Each drum support preferably includes a strengthening ribarea 23 and a drum accommodating well area 25 as depicted to accommodatethe respective drum end 21, 22 of the drum 1. The drum supports 16engage with sub-structure by interlocking within complementary surfacesprovided in side walls 27,28. Other less preferable constructions arepossible, such as frameworks formed from individual members ormechanical suspension systems.

The drum supports 16 each include a bearing support well at the centreof said well area 25. A bearing mount 29 is located within the bearingsupport well, and in turn the bearing 15 fits within a boss in thebearing mount 29.

In the preferred embodiment of the invention, as shown in more detail inFIGS. 3 and 4, the drum 11 comprises a perforated metal hoop 30, a pairof ends 21, 22 enclosing the ends of the hoop 30 to form a substantiallycylindrical chamber and a pair of vanes 31 extending between the drumends 21, 22.

In the preferred form of the invention the drum is driven only from oneend 21 and consequently one purpose of the vanes 31 is to transmitrotational torque to the non-driven drum end 22. The vanes also providelongitudinal rigidity to the drum assembly 11. To these ends the vanes30 are wide and shallow, although they have sufficient depth andinternal reinforcing to achieve any required resistance to buckling dueto unbalanced dynamic loads. Preferably the vanes 30 have a distinctform, including a leading and trailing edge to assist in tumbling thewashing load. In the preferred embodiment the vanes 30 are orientedoppositely in a rotational direction, so that under rotation in eitherdirection one vane is going forwards and the other backwards. This vaneconfiguration provides further benefits in providing a user friendlyopening into the washing chamber as is described below.

In the preferred embodiment of the washing machine incorporating theinvention the drum 11 is supported between a pair of drum supports 16one at either end thereof. Access to the interior of the drum 11 isprovided through a slide away hatch section 33 in the cylindrical wall30 of the drum. The hatch section is connected through a latchingmechanism 34, 35, 36, 37, 38 such that it is connected in a continuousloop during operation. Accordingly the cabinet 12 of the washing machineis formed to provide access to the drum 11 in a substantially toploading fashion, rather than the traditional front loading fashion morecommon to horizontal axis machines.

The washing machine includes an electric motor (rotor 39 and stator 40visible in FIG. 4) to effect rotation of the drum during all phases ofoperation (wash, rinse and spin dry). In the preferred form of thewashing machine incorporating the present invention the motor is adirect drive inside-out electronically commutated brushless dc motorhaving a permanent magnet rotor 39 coupled to one end 21 of the drum 11and stator 40 coupled to the drum support 16. A suitable form of motoris described in EP0361775.

A user interface 24 is provided, allowing user control over thefunctions and operation of the machine. The control electronics areintegrally contained within the interface module, and provide electroniccontrol over the operation of the machine.

Balancing System

In the present invention the forces caused by an out-of-balance loadduring high speed rotation of drum 11 to affect spin drying areminimised by a dynamically controlled balancing system. This balancingsystem uses electrical signals generated by the deformation of loadcells in the bearing mounts 29 at each end of the shaft 19 to assess therequired weight distribution correction that is required to dynamicallyrebalance the drum 11. Each bearing mount 29 is formed with a pair ofbending bridges 40,41 and mounted on each bending bridge is a load cell42 as shown in FIG. 5. The outputs of the load cells 42 are fed to thecontrol processor of the laundry machine to effect the balancing task,which is achieved by the addition of water to one or more of the sixbalancing chambers 43,46,47,80,81,82 located in the drum, as shown inFIG. 6. There are three such chambers at each end spaced 120° apart andpositioned on the extremity of the drum end 21,22.

In more detail the balancing system is illustrated in FIG. 7. The outputfrom the load cells 42 is first passed through filtering 50 beforeconnection to the inputs of a microprocessor 51, which may be taskspecific or the main control processor for the laundry machine. Thevarious algorithms (detailed later) programmed into the microprocessor51, will dictate spin commands (eg: speed up/slow down) to the motorcontroller 52 and balancing corrections (eg: open/close valve 54) to thevalve driver 53. The motor controller 52 in turn, will vary itsenergisation of the motor windings to achieve the spin command. Thevalve driver 53 will open or close the appropriate balancing valve 54,which allows water to flow through the injector 44 into the relevantslot 45 whereupon it is channelled to the appropriate chamber. The valvedriver 53 also allows switching between coarse and fine control modes byswitching the water flow through the high 55 and low 56 flow rate valvesrespectively.

To correct an imbalance, it is necessary to artificially add equal andopposite static and dynamic imbalances. To add a static imbalance onlyrequires to add a certain amount of mass at some radius and rotationangle (or ‘phase’ angle), at the same location along the spin axis asthe CoG. However, to add a dynamic imbalance requires to add two equaland opposite imbalances at two locations along the spin axis that areevenly spaced either side of the CoG. The end result is that both staticand dynamic imbalances can be corrected by adding, at two separatelocations along the spin axis, two independent masses (both may be atthe same radius) at two independent phase angles. There are fourvariables to be defined, and so four useful pieces of information aboutthe nature of the imbalance must be obtained.

These pieces of information are typically obtained by measuring eitheracceleration, velocity, force, or displacement at two independentlocations on the vibrating system. The reason that only two sensorlocations are required and not four is that because the relevant signalsare sinusoidal in time and therefore contain two pieces of information.One is the magnitude of the signal, and the other is the “phase” anglewith respect to some reference point on the spinning system.

Once the signal magnitude and phase angle at two independent locationsare acquired, a method is required to calculate the two masses and theirphase angles with which to correct the imbalance. This is done byrepresenting the signal data and mass data as vectors of two complexnumbers, and the relationship between them as a square matrix of fourcomplex numbers. This matrix, when for mapping the mass vector to thesignal vector, is called a response matrix, and it is its inverse thatis used to map the signal vector back to the mass vector representingthe imbalance.

The technique for acquiring data on the imbalance is difficult toimplement in practice. This is because some types of signal are moredifficult to measure than others, and even if good signals are obtained,the response matrix can become a unpredictable and difficult thing toknow (or learn) depending where the signals are measured. In thepreferred embodiment of the present invention the imbalance ischaracterised using force or stress measurement. Of the availablealternatives force is easy to measure and the signal level is quiteadequate at low speeds.

Because the machine has no suspension the cabinet is effectively rigidlyconnected to the spin axis of the drum. This means that the responsematrix that relates imbalance to force at the bearing assemblies isreasonably diagonal and does not vary in a complex and/or unpredictablemanner with speed where the appliance is supported on a rigid floor.Thus a radial component of force (vertical for instance) at the bearingassemblies at each end of the drum, is the most useful signal to measurefor the purpose of balancing, with a rigid floor. Where the floorsupporting the appliance is flexible a different relationship applies,which is discussed later.

Sensors

To perform a complete static and dynamic balance requires four usefulpieces of information to be known about the nature of the imbalance. Ithas also been shown that the desirable signals for the purpose ofbalancing are a radial component of force at each bearing assemblysupporting the drum, and thus two load cells of some sort are required.In the preferred embodiment a pair of sensors 42 are located at eitherend of the shaft 19 as shown in FIG. 4.

A strain sensor suited to this application is the piezo disc. This typeof sensor produces a large signal output and so is not significantlyaffected by RFI. However a piezo strain sensor can only measurefluctuations in load due to charge leakage across the disc.

The piezo disc will have a particular response in relation to appliedforce. Since force is proportional to frequency squared and the responsemagnitude is proportional to force frequency, the relationship betweensensor output and rpm of the drum is cubic.

In more detail the bearing mount looks like two concentric cylindricalrings 46, 47, as illustrated in FIG. 5. The load bridges 40, 41described previously are connected at the top and bottom of the innerzing 47, respectively, and to opposite parts of the upper periphery ofthe outer ring 46. A piezo disc 42 is adhered to the loading bridge ontothe side facing the outer ring. The load from the drum is taken througha bearing 15 mounted in the internal ring 47, through the load bridges48 and load cell 42 into the outer ring 46, and out into the externalstructure. It will be appreciated that in this fashion the load bridgeswill flex according to any vertical forces from the spinning of thedrum, thus deforming the piezo disc and providing a signalrepresentative of the imbalance force.

Dynamic Control

In the preferred embodiment of the invention a dynamic control method isused. This is not in any way to be confused with static and dynamicimbalance as explained earlier, it simply refers to the nature of thecontrol methodology. The alternative control methodology is ‘static’. Astatic control method does not make use of or retain data on the timedependent behaviour of its target system. As a result the method isexecuted as a ‘single shot’ attempt to restore equilibrium, andsufficient time must be allowed to lapse after each execution so thatthe system has returned to a steady state condition prior to the nextexecution. Whereas a dynamic control method can anticipate the timedependent behaviour of the system and by storing recent past actions itis able to continuously correct the system, even while the system is intransient response.

The main advantage of the preferred dynamic control is that the controlloop is able to adjust for discrepancies as and when they appear ratherthan having to wait for the next execution time to come round. Forsystems with slow time response this is a considerable advantage. Towork effectively the controller must be programmed with an estimate ofthe time dependent response of the target system. However, provided ithas no significant quirks, this only needs to be roughly approximatedand the approach will still work well, Also, because the dynamiccontroller runs on a fast decision loop, any noise on the inputparameters will result in many small corrections being made that arecompletely unnecessary. For this reason a minimum threshold correctionlevel must be established where there is any cost or difficultyassociated with effecting a correction.

Listing the main sources of time dependent behaviour:

Given an instantaneous change in balance state of the machine, it willtake a few revolutions to reach a steady state of vibration.

The forgetting factor averaging on the load cell data acquisition meansthat the averaged data also takes a number of revolutions to respond toa new vibration state.

Change in balance state of the machine is never instantaneous; wateraddition requires anything from 0.1 to 60 seconds.

Water extraction from the load means the balance state of the machinemay change quite rapidly as its spin speed ramps up.

If in the spin cycle the machine is to ramp from 100 to 1000 rpm inabout 3 min then the machine will almost certainly be in a state oftransient response for the duration of this period. Consequently thecontroller must be able to respond to changes in the balance state ofthe machine without the machine ever being in a steady state condition.

As previously stated for dynamic control to be implemented the presentcontroller must be programmed with an approximation of the timedependent behaviour of the machine. More precisely it must know how muchto weight its past actions (as a function of how long ago they weremade) when deciding on what corrections, if any, are to be implemented.In this application, for each water chamber the sum of the appropriatelyweighted past history of water addition can be considered to be ‘Effectin Waiting’; i.e. the controller is still anticipating that the effectof a certain quantity is still to come through on the signals, and thusmust subtract his ‘Effect in Waiting’ from the presently calculatedwater requirements when deciding which valves should be on and whichshould be off at present.

To do this accurately requires a complete record of the controllers pastactions for as many points back as it needs to remember, and a table ofweighting values for as many points, which in this application will beat least ten. If we call this number of points N, then to store thehistory of six control output channels with N points each requires 6Ndata points. Also, to then calculate the effect of this history willrequire 6N multiplications. One simplification would be to approximatethe exact weighting curve 60 with a ‘table top’ curve 61 as shown inFIG. 9. This then eliminates the need for a stored table of weightingvalues, and reduces the 6N multiplications to 6N additions, but eventhis is still to complicated. A very crude approximation of the exactweighting curve is the negative exponential 62 also shown in FIG. 9.While this sounds complicated it is in fact extremely easy to achieve,it is simply a forgetting factor type average. All that needs to be doneis this: for each water control channel, create an effect in waitingvariable and each time the control loop executes multiply it by acertain factor (between zero and one) and add to it some increment valueif the water control valve for this channel was on during the last loop.Computationally all that is required is six multiplications and sixadditions with each control loop execution; a vast saving. To avoid theneed to have different forgetting factors dependent on speed, thecontrol loop must be executed on a per revolution basis. This is simplyachieved by executing the balance control code with the once perrotation sensor, directly after the data acquisition conversion code. Ofcourse all quantities of water must now be calculated in terms ofrevolutions at the present speed rather than time, but this is a simplematter in that the magnitude calibration factor will now vary like rpmrather than rpm squared.

Another point to consider is that, considering one end at a time, if theout of balance load is directly opposite one of the chambers (saychamber number 43) then the data acquisition routine will identify thischamber as the primary one needing water, however, due to noise on thesignals, it will almost certainly also say that one of the otherchambers needs a small amount of water as well. This second waterrequirement will be much smaller than the other one and will sometimesbe chamber 46 and sometimes chamber 47 depending on just what the noisewas in the last few revolutions. If the balance control routineaddresses these secondary small water requirements then over therelatively long period of addressing chamber 43 it will also graduallyfill chambers 46 and 47, thus negating some of the water going intochamber 43, and leaving less headroom for further balancing correctionslater on. Clearly the balance controller must not address two chambersat once at one end unless it is clear that neither of them could be dueto noise, i.e. both of them require a similar amount of water. Similarlybecause the ends of the machine are not truly independent systems butare weakly coupled (as will be discussed later) then large out ofbalance forces at one end cause ‘ghost images’ at the other, thus thebalance controller must not address two ends at the same time unless itis clear that neither of them could be ghost images, i.e. both endsrequire a similar amount of water. The easiest way to address both ofthese problems is identify the maximum water requirement out of the sixchambers and to then set a dynamic ‘noise’ threshold equal to half ofthis value of water (as shown in FIG. 10). A water valve (e.g. 5) isthen only turned on if the result 72 of its present requirements 70,minus its present effect in waiting 71, minus the noise value, isgreater than the increment value mentioned above. It is here that weperform our magnitude calibration by adjusting this increment value.

Finally, a small amount of hysteresis is necessary to prevent repetitiveshort valve actuations. This is simply achieved by using the abovecriterion for deciding when to run a valve on, but using a differentcriterion when deciding when to turn it off again. The off criterion ismore simple: a water valve is only turned off once its presentrequirements is less than its present effect in waiting. In other wordsonce the valve is on it is not turned off until its chamber requirementsare addressed.

Control Algorithms

The task of spinning while balancing actively can be subdivided intothree sub-tasks or algorithms:

Imbalance Detection Algorithm (IDA)

Balance Correction Algorithm (BCA)

Spin Algorithm (SA)

The Imbalance Detection Algorithm (IDA) (shown in FIG. 11) is concernedsolely with the acquisition of imbalance related data, and is embeddedin the motor control routine. It is active whenever the motor isturning, and makes its results available for the Balance CorrectionAlgorithm (BCA) to see.

The Spin Algorithm (SA) (shown in FIG. 13) is concerned solely withexecuting the spin profile asked of it. It ramps the speed of themachine according to the profile requested and the vibration leveldetermined by IDA.

BCA (shown in FIG. 12) is concerned solely with correcting whateverimbalance IDA has determined is there. It is an advanced controlalgorithm that takes into account the time dependent behaviour of boththe machine and IDA. BCA is active whenever the rotation speed of themachine is greater than approximately 150 rpm.

Signal Analysis—IDA Processing

To determine the imbalance in the load requires the magnitude and phaseangle of the once per rotation sinusoidal component in each of thesignals. Unfortunately the signal does not look like a clean sinusoid,but is messy due to structural non-linearities in the machine as well asRadio Frequency Interference (RFI). The once per rotation component or‘fundamental component’ must be somehow obtained out of such a signal.

This is done by digitally sampling the signal and using the discreteFourier Transform technique. It is not necessary to compute an entiretransform, which would give us half as many frequency components as wehave signal samples inside of one revolution (and would also take sometime in an 8-bit microprocessor), but just the fundamental component.The way this is done is to multiply each of the signal data pointsobtained by the value of a once per rotation cosine wave at theequivalent phase angle lag after the rotational reference mark, and sumeach of these results over a whole revolution, and then divide by thenumber of results. This gives the real (or x) component of the complexnumber result. The imaginary (or y) component is derived using the sametechnique but using a sin wave instead of a cosine wave. The resultingcomplex number may then be converted in polar form, giving magnitude andphase angle of the fundamental component in the signal. Also to preventaliasing the input signal is passed through an analogue filter first toremove frequency components higher than half of the sampling frequency.

The discrete Fourier analysis may be made considerably more simple ifthe sampling is performed using a fixed number of samples per revolutionrather than a fixed frequency. This of course requires a rotary encoder,which in this application is already provided in the form of a DCBrush-less motor. It is therefore necessary to use a number of pointsper revolution that divides exactly into the number of commutations perrevolution executed by the motor. This also enables the sine values thatwill be required to be pre-programmed as a table (termed the ‘sinetable’), from which the cosine values may be obtained by offsettingforwards by a quarter of the number of samples per period. It isnecessary to have a reasonable number of sampling points per revolutionso that the order of harmonics that are aliased onto the fundamentalcomponent is well beyond the cut-off frequency of the low pass filter.This means that the number of sampling points must be at least 12 toobtain reliable sampling at speeds upwards of 200 rpm. An even number ofpoints per revolution for sampling should be used so that the sine tableis perfectly symmetrical, i.e. the positive sequence and the negativesequence are identical apart from their sign. This ensures that the DCoffset on the input signal does not influence the fundamental component.FIG. 8 illustrates the signal after filtering 57 and the extractedfundamental component 58.

Alternatively, if a more powerful microprocessor is employed then bymaximising its data acquisition capabilities the noise problem will befurther reduced. This would mean instead of fixed sampling on a perrevolution basis, it would be on a fixed frequency basis—at a higherrate. Further, the sine and cosine valves could be either calculated orinterpolated from a table, which simplifies much of the calculations.

Once the fundamental component of the source signals is obtained it willinevitably contain some noise component (i.e. consecutive measurementswill have some variance). The best way to get rid of this is to ensurethat the signal source is accurate, clean, and has linear response. Oncethe source end has been addressed then averaging techniques may be usedto address the remainder of the noise.

One such technique is to implement a ‘Forgetting Factor’. This is whereevery time a new measurement is acquired the new average is equal to forexample 70% of the old averaged value plus in this case 30% (=100%−70%)of the new measurement. Here the forgetting factor used was 0.3 since0.3 of the old average is forgotten and replaced it with 0.3 of the newmeasurement. This form of averaging suits microprocessor basedapplication since it is inexpensive with respect to both memory spaceand processor time.

The main disadvantage with averaging the measurements is that theresponse time of the imbalance detection goes down. This is simply aresult of the fact that the averaged result must incorporate severalmeasurements in order to reduce the noise, which of course can only beobtained from past measurements, not future ones. The lower theforgetting factor, the more the averaged value remembers from pastmeasurements, and thus the slower it responds to a change in themachine's vibration.

Because the balancing can only be executed over many iterations (due towater extraction from the load) it is not necessary to be able to obtaina perfect balance in one ‘hit’. From this point of view it is thenacceptable to make a few ‘approximations’, the biggest of which is totreat the machine as two independent single degree of freedom (SDOF)systems associated with each signal source. The main advantage of doingthis is that the micro does not have to calculate and invert the 2×2response matrix, it only has to estimate the two SDOF responses for eachend.

Since the measurement data are complex numbers in Cartesian format (x &y), whereas the responses are in polar format (magnitude & phase), aformat conversion and complex division is required at each end to obtainthe water correction vector While this is not impossible to executeconventionally, there is a more simple approach: take the phases of theresponse and incorporate them directly into the discrete Fouriertechnique as offsets each of an integer number of points whenreferencing the table of sine values. These offsets may then adjusted asthe machine changes speed for phase angle calibration. Alternativelyphase calibration may be performed using a rotation matrix acting on thevectors as calculated without any applied offset to the sine table.Magnitude calibration however, is performed later in the dynamic controlroutine.

Once having obtained the x and y components of the imbalance at each endof the drum, it is then required to calculate how much water eachchamber at each end needs since the chambers are 120 degrees apart. Ifthe chambers were 90 degrees apart, (i.e. orthogonal like the x and yaxes) then the problem would be trivial, but this would require fourchambers for each end and thus two more water control valves andassociated drivers than necessary. A more simple approach is tocalculate the projection of the signal vector onto axes that are 120degrees apart, the same as the chambers.

The way to implement this is very simple. The Fourier technique usessine and cosine wave forms to extract the orthogonal x and yprojections. This follows quite naturally from the fact that a cosinewave is a sine wave that is has been shifted to the left by 90 degrees.Therefore to split the signal vectors into projections that are 120degrees apart simply requires to replace the cosine wave form with asine wave form that has been shifted to the left by 120 degrees, i.e.one third of a rotation.

The phase calibrated signals now represent the projection of theimbalance onto the first two chambers. To obtain the projection of the,imbalance onto the third chamber to be may use the vector identity thatthe sum of three vectors of equal magnitude and all spaced 120 degreesapart must be equal to zero. Hence the sum of all three projections mustbe zero, i.e. the projection onto the third chamber is the negative ofthe sum of the projections onto the first two chambers. By adding half arotation to the response phase angles the three values obtained are madeto represent the projection of the restoring water balance required ontoeach balancing chamber.

Finally, at least one of these three projections will be negative,representing water to be removed from that chamber. This cannot be doneand so we simply add a constant to all three numbers so that the mostnegative number becomes zero and the other two are guaranteed positive.

Overall Control Strategy—SA

The overall control over the spin process is assigned to the spinalgorithm SA. It begins with the bowl speed at zero, and disables theBCA. Its first task is to better distribute the wash load to allowspinning to begin. If at a very low spin speed the vibration is belowthe initial threshold, it is allowed to spin to the minimum BCA speed atwhich point BCA is enabled. If the vibration is not below the threshold,redistribution is retried a number of times before stopping anddisplaying an error message. Once BCA has attained the target level ofspin speed the spin is allowed to continue for the desired period afterwhich the bowl is stopped, valves are closed and BCA is disabled.

Dynamic Balancing—BCA

In more detail the balance correction algorithm shown in FIG. 12 beginswith calibration of the phase information from the IDA. The step ofvector rotation is optional depending on the method used (alternative isto apply in offset to the sine table). Following this the vectors arenormalised and the level of vibration is calculated. If the enable flagis true and the level of vibration is below a predefined critical limitthe decision making process begins. Firstly the vibration level iscompared to a number of threshold values to assess whether to enableincrease of the bowl speed. Then depending on the level of vibrationfine or coarse (low or high flow rate to valves) correction is enabled.The effect in waiting of past actions is then updated, and together withthe current vector information and the status of each valve a decisionis made whether to open or close each valve. Then if the hold bowl speedflat is not enabled i.e. acceleration is allowed, and the speed is notcurrently at the desired target level, the bowl speed is allowed toincrease to the target level. At this point it loops to the start andbegins another iteration, effectively continuously correcting andaccelerating until it reaches the target speed.

Further Improvements

It will be appreciated in the preceding embodiments that the washingmachine is assumed to be supported on a rigid surface such as a concretefloor. Where this is not the case, for example, wooden floors, and theentire washing machine is permitted substantial displacement during thespin cycle, then those techniques previously described will not beentirely successful. Therefore, in a further improvement the presentinvention also provides a method and apparatus for correcting for spinimbalances when the washing machine is supported on a non-rigid supportsurface.

The equivalent spring system which represents the spin drum 100, themachine frame 102 and the reference surface is shown in FIG. 14. Thefirst spring 106 between the spring drum 100 and the machine frame 102effectively represents the elasticity of the load bridge which connectsthe bearing mount to the drum support or frame of the washing machine.This bridge also forms the basis of the load cell which measures theforces between the drum and the frame of the washing machine. The secondspring component 108 in this case represents the elasticity of thesupport surface, for example, flexible wooden floorboards. The secondspring 108 is complex and includes a damping component 110. In order tomeasure the acceleration or displacement of the drum 100 relative to thereference surface 104, i.e. a stationary reference point, aaccelerometer 112 is connected either to a non-rotating part of thebearing itself or on an adjacent section of the load cell bridge.

Now, consider that the machine is spinning at a particular speed and isin a perfectly balanced state. Suppose we now add a small “Out OfBalance” (F_(O/B)) load at one end (by injecting some water into one ofthe balance chambers). If the ends of the machine behaved entirely asindependent mechanical systems then we would expect that we would nowmeasure a force vector at the end to which we added water, and thatnothing would change at the other end: the other end would remainperfectly balanced. However, the ends of the machine are not independentsystems, and in reality we find that we now measure a force vector atboth ends of the machine. The two ends are said to be ‘coupled’together. As a result of this coupling, the observed force vector at oneend of the machine is related not-only to the “out of balance” F_(O/B)vector at the same end, but it is also related to the F_(O/B) vector atthe other end of the machine. Thus:

F ₁ =R ₁₁ *F _(O/B1) +R ₁₂ *F _(O/B2)

Where F₁ is the force vector measured at one end 1 of the machine,F_(O/B1) and F_(O/B2) are the F_(O/B) vectors at ends 1 and 2respectively and R₁₁ and R₁₂ are the individual response factors thatF_(O/B1) and F_(O/B2) have at end 1. (Note that R₁₁ and R₁₂ are alsovectors; each consisting of magnitude, and phase lag of the response)

Similarly at end 2 we may write

F ₂ =R ₂₁ *F _(O/B1) +R ₂₂ *F _(O/B2)

Where F₂ is now the force vector as measured at end 2 and R₂₁ and R₂₂are the individual response factors that F_(O/B1) and F_(O/B2) have atend 2.

These two equations may be mathematically combined as a Matrix equation:

F=R*F _(O/B)

Where F is the column vector (of vectors) $\begin{bmatrix}F_{1} \\F_{2}\end{bmatrix}\quad $

F_(O/B) is the column vector (of vectors) $\begin{bmatrix}F_{O/{B1}} \\F_{O/{B2}}\end{bmatrix}$

R is the response matrix (of vectors) $\begin{bmatrix}R_{11} & R_{12} \\R_{21} & R_{22}\end{bmatrix}$

Now, if the machine held the bowl absolutely rigid while spinning thenwe would expect the force transducers to measure precisely the forcevectors required for the centripetal acceleration of the F_(O/B) loadvectors. But this is not the case. The external structure of the machineis not infinitely stiff, and neither is the floor, the house, or eventhe ground under the house for that matter. As a result the forcetransducers also measure a component due to the mechanical response ofthe machine which is a function of all of the above (machine structure,floor, house . . . ), and also of bowl rotation speed. Note that it isthis extra component of machine response that makes the coupling termsin the matrix (R₁₂ and R₂₁) significant and the whole matrix in generalimpossible to pre-calibrate.

It is here that two possible techniques emerge:

1) By measuring acceleration vectors at each end we may determine themachine's mechanical response, and then by appropriately combining forcevector and acceleration vector at each end we can make a new vectorquantity for which the response matrix is uncoupled (i.e. R₁₁ and R₂₂are the only significant terms). Further the matrix is not a function ofunknown parameters and thus can be factory calibrated.

2) Or by making small, but known, changes to the F_(O/B) vectors andmeasuring the resultant change in force vectors, it is possible to learnthis response matrix ‘R’ during the spin cycle.

The first technique is very robust, but requires the addition ofacceleration sensors to measure absolute vertical acceleration of thedrum.

The second technique is very clever, but has several difficultiesassociated with it which are outlined further on.

First Method—Acceleration Measurement

From the system described above it will be apparent that the forcemeasured by the load cell will not be an accurate measure of theimbalance. In order to determine the imbalance to correct the controllermust take account of the effect of the complex system external to thatof the washing machine. It will be appreciated therefore that theabsolute force F_(a) acting on the spin bowl can be expressed as

F _(a) =m ₁ ×a _(a)

where m₁ is the mass of the spin drum and a_(a) is the absoluteacceleration of the drum, as measured by the accelerometer. This forcein turn is then composed of:

F _(a) =F _(o/b) +F ₁

where F_(o/b) is the out of balance force and F₁ is the force measuredby the load cell. By rearrangement the out of balance force F_(o/b) maybe expressed in terms of known variables

F _(o/b)=(m ₁ ×a _(a))−F₁

and calculated by the controller. Whereas F₁ would be available from IDAas previously described, the output of the accelerometer would need tobe put through a similar filtering process to the IDA, in order toprovide a useful signal. The drum mass m₁is estimated based on the knownweight of the drum, the amount of water added to the load and knowncharacteristics of the load based on the “type” of load. The “type” ofload may be determined using any one of a number of well known fabricsensing techniques such as that disclosed in our U.S. Pat. No.4,857,814.

The above makes the assumption that each end of the drum may be treatedseparately. We have found that by using this method this is asatisfactory assumption. However in some cases this may not be adequateand therefore a more accurate system may be required. In this case it isnecessary to take into account the coupling between each end of thedrum. To this end a coupling matrix γ may be determined by successivetests on the system, where ξ is the ratio of the position of the centreof gravity to the length of the drum, and α is the inertia factor.$\begin{matrix}{\alpha = \frac{I}{m_{1}l^{2}}} \\{\underset{\_}{\gamma} = \begin{bmatrix}{\left( {1 - \xi} \right)^{2} + \alpha} & {{\left( {1 - \xi} \right)\xi} - \alpha} \\{{\left( {1 - \xi} \right)\xi} - \alpha} & {\xi^{2} + \alpha}\end{bmatrix}}\end{matrix}$

from this we may calculate the out of balance force:

F _(o/b) =γm ₁ A+F ¹

where the acceleration vector A may be represented$\underset{\_}{A} = \begin{pmatrix}a_{1} \\a_{2}\end{pmatrix}$

and the force measure of the load bridge F₁ as$\underset{\_}{F_{l}} = \begin{pmatrix}F_{l1} \\F_{l2}\end{pmatrix}$

Second Method—Determining the System Response

Whereas previously:

F=R*F _(O/B)

If the response of the machine is relatively linear

dF=R*dF _(O/B)

Where dF and dF_(O/B) are still 2*1 column vectors, and R is the 2*2response matrix. dF represents the change in the force vectors as aresult of adding F_(O/B) vectors dF_(O/B). However, in the real world wewill want to find out the F_(O/B) vectors needed to remove the F vectorsmeasured. To do this we need to rearrange by multiplying each side bythe inverse of R:

inv(R)*dF=inv(R)*R*dF _(O/B)

Yielding

dF _(O/B)=inv(R)*dF

Since any matrix times it's inverse gives the identity matrix. Let usalso call the inverse of R ‘A’ since it is really the ‘action’ matrixthat tells us what to do given what we measure. Thus:

dF _(O/B) =A*dF

Where A=inv(R)

The problem is we want to find out A. The way to do this is to add asmall, but known, additional imbalance to one end and nothing to theother. Let us denote the addition as dF_(O/Ba), and the correspondingchanges in the force vectors as dF_(a). Remember dF_(O/Ba) and dF_(a)are both column vectors (of vectors). Now repeat the exercise but thistime adding another small addition to the other end. This time let usdenote the addition as dF_(O/Bb), and similarly the corresponding changein force vectors dF_(b). Now we can combine the two experiments togetherto write:

(dF _(O/Ba) dF _(O/Bb))=A*(dF _(a) dF _(b))

Or

 DF _(O/B) =A*DF

Where DF_(O/B) and DF are now the 2*2 matricies formed by joining two2*1 column vectors side by side. Multiplying each side of the equationby the inverse of DF:

DF _(O/B)*inv(DF)=A*DF*inv(DF)

Yielding

A=DF _(O/B)*inv(DF)

And thus the action matrix is now known, and may be used to calculatethe correction required eliminating the measured F vectors. Toillustrate all this here is a worked example. Suppose the machine inpresently spinning at some constant speed, and the force vectors wemeasure at each end are: $F = \begin{bmatrix}{1{\angle 0}} \\{2{\angle 90}}\end{bmatrix}$

Now suppose we add one unit of water at 90° at end 1, and nothing at end2, and the new force vectors become: $F_{new1} = \begin{bmatrix}{1.414{\angle 45}} \\{2.236{\angle 53}{.4}}\end{bmatrix}$

This gives: ${dF}_{O/{Ba}} = {{\begin{bmatrix}{1{\angle 90}} \\{0{\angle 0}}\end{bmatrix}\quad {and}\quad {dF}_{a}} = \begin{bmatrix}{1{\angle 90}} \\{1{\angle 0}}\end{bmatrix}}$

Now for the second run suppose we add 0.5 units of water at 0° at end 2,and nothing at end 1, and the new force vectors become:$F_{new2} = \begin{bmatrix}{2.414{\angle 45}} \\{3.200{\angle 57}{.7}}\end{bmatrix}$

This gives: ${{dF}_{O/{Ba}} = {{\begin{bmatrix}{0{\angle 0}} \\{0.5{\angle 0}}\end{bmatrix}\quad {and}\quad {dF}_{a}} = \begin{bmatrix}{1{\angle 45}} \\{1{\angle 45}}\end{bmatrix}}}\quad$$\quad {{{Thus}\quad {DF}_{O/B}} = {{\begin{bmatrix}\begin{matrix}{1{\angle 90}} \\{0{\angle 0}}\end{matrix} & \begin{matrix}{0{\angle 0}} \\{0.5{\angle 0}}\end{matrix}\end{bmatrix}\quad {And}\quad {DF}} = \begin{bmatrix}\begin{matrix}{1{\angle 90}} \\{1{\angle 0}}\end{matrix} & \begin{matrix}{1{\angle 45}} \\{1{\angle 45}}\end{matrix}\end{bmatrix}}}\quad$${{{Thus}\quad {{inv}({DF})}} = \begin{bmatrix}\begin{matrix}{0.707{\angle 225}} \\{0.707{\angle 0}}\end{matrix} & \begin{matrix}{0.707{\angle 45}} \\{0.707{\angle 270}}\end{matrix}\end{bmatrix}}\quad$ ${{{And}\quad {so}\quad A} = \begin{bmatrix}\begin{matrix}{0.707{\angle 315}} \\{0.354{\angle 0}}\end{matrix} & \begin{matrix}{0.707{\angle 135}} \\{0.354{\angle 270}}\end{matrix}\end{bmatrix}}\quad$

With A now calculated and knowing F as measured by the load bridge, therequired correction to counteract the imbalance can be calculated.Intially the action matrix is completely unknown thus we must makerandom guesses for the inital F_(O/B) vectors. After we have someknowledge of the matrix we may make better guesses for the initialF_(O/B) vectors.

Overall System Advantages

The advantages for the Washing Machine of employing and active balancingsystem are:

Forces due to imbalance are eliminated prior to bearing assemblies. Thusstructural requirements are reduced, enabling less and/or cheapermaterial to be employed.

Suspension which wears out and deteriorates is eliminated.

Wash cylinder clearances reduced enabling ample load capacity in amachine of standard size.

Complexity of door opening mechanism also reduced because it no longerneeds to cope with height changes on a suspension.

Quiet smooth spinning at all times.

Able to cope with variable external conditions.

What we claim is:
 1. A laundry appliance having a perforated drum fordehydrating a clothes load, an electric motor adapted to rotate saiddrum at speed thereby dehydrating the load and a system for compensatingfor imbalances of said drum and any load carried therein duringdehydration of the load, said system comprising: at least one sensorlocated at more than one position on the drum spin axis for detectingrotational imbalance in the load, correction means for adding one ormore masses to said drum, a digital processor which in use receives asinputs signals from said sensor, and programmed to calculate the valueand position of one or more masses required to be added to the drumdepending on at least said sensed imbalance and any past, current orfuture additions, said processor controlling such additions such thatthe resultant value and position is substantially similar to saidmodified value and position to correct the imbalance.
 2. A laundryappliance having a perforated drum for dehydrating a clothes load,driving means adapted to rotate said drum at speed thereby dehydratingthe load and a system for compensating for imbalances of said drum andany load carried therein during dehydration of the load, said systemcomprising: at least one sensor located at more than one position on thespin axis of said drum for detecting rotational imbalance in the load,correction means for adding two or more masses to said drum to correctfor any imbalance caused by the rotation thereof, and a digitalprocessor which in use receives as inputs signals from said sensor andprogrammed with software causing said processor to carry out thefollowing steps: a) energising said electric motor to apply a firstpredetermined rate of rotation to said drum; b) energising saidcorrection means to add at least one small imbalance to at least one endof said drum and storing the detected rotational imbalances at each endof said drum; c) determining the differential relationship between saidat least one added imbalances' and said detected rotational imbalances'at each end of said drum, thereby estimating the value and position ofone or more masses required to be added to the drum to correct theactual imbalance; and d) controlling additions of one or more masses tosaid drum by said correction means such that the resultant value andposition of the added masses is substantially similar to the saidestimated value and position to correct the imbalance.
 3. A laundryappliance as claimed in claim 2 wherein said step (b) comprises thefollowing steps: b.1) energising said correction means to add a firstsmall imbalance at one end of said drum; b.2) storing the detectedrotational imbalances at each end of said drum as a first measuredimbalance and a second measured imbalance respectively; b.3) energisingsaid correction means to add a second small imbalance at the other endof said drum; and b.4) storing the subsequently detected rotationalimbalances at each end of said drum as a third measured imbalance and afourth measured imbalance respectively.
 4. A laundry appliance having aperforated drum for dehydrating a clothes load, driving means adapted torotate said drum at speed thereby dehydrating the load and at least twoindependent systems for compensating for imbalances of said drum and anyload carried therein during dehydration of the load, each said systemcomprising: at least one sensor located at one or more positions on thespin axis of said drum for detecting rotational imbalance in the load, adigital processor which in use receives as inputs signals from saidsensor and programmed to estimate the value and position of one or moremasses required to be added to the drum to correct the sensed imbalance,correction means for adding one or more masses to said drum, saidprocessor in use controlling such additions such that the resultantvalue and position of the added masses is substantially similar as thesaid estimated value and position to correct the imbalance.
 5. A laundryappliance according to claim 4, wherein said sensor and said correctionmeans, for each respective system, are substantially proximate eachother and/or there being a substantially rigid connection therebetween.6. A laundry appliance having a perforated drum for dehydrating aclothes load, an electric motor adapted to rotate said drum at speedthereby dehydrating the load and a system for compensating forimbalances of said drum and any load carried therein during dehydrationof the load, said system comprising: at least one sensor located at morethan one position on the drum spin axis for detecting rotationalimbalance in the load, correction means for adding one or more masses tosaid drum, a digital processor which in use receives as inputs signalsfrom said sensor and adapted to energise said electric motor and saidcorrection means and programmed with software causing said processor tocarry out the following steps: 1) monitor the rotational imbalance basedon the output of said sensor; 2) energise said electric motor toredistribute the load within said drum if said estimated imbalance isabove a first predetermined threshold; 3) if said estimated imbalance isbelow said first predetermined threshold determine the value andposition of one or more masses required to be added to the drum tocorrect the sensed imbalance; 4) if said estimated imbalance is belowsaid first predetermined threshold energise said correction means suchthat the resultant value and position of any additions is substantiallysimilar to the calculated value and position to correct the imbalance;and 5) energise said electric motor to apply a further faster rate ofrotation to said drum so as to effectively dehydrate said load.
 7. Alaundry appliance according to claim 6, wherein said step (5) furthercomprises the following steps: 5.a) estimating the rotational imbalancebased on the output of said sensor; 5.b) if said estimated imbalance isbelow a second predetermined threshold, energising said electric motorto increase the rate of rotation of said drum by a predeterminedincrement; 5.c) if said estimated imbalance is above a secondpredetermined threshold, calculating a corresponding correction tocounteract said imbalance; 5.d) if said estimated imbalance is above asecond predetermined threshold, then adding one or more masses to saiddrum using said correction means, corresponding to said calculatedcorrection; and 5.e) if the rate of rotation of said drum is below thelevel for effective dehydration of the load, then repeating steps (5.a)to (5.d).
 8. A laundry appliance as claimed in claim 7 wherein saidprocessor includes storage means adapted to store data including apredetermined number of said past corrections made by said correctionmeans; and said step 5.c) comprises the following steps: 5.c.i) if saidestimated imbalance is above a second predetermined threshold,estimating the steady state rotational imbalance based on the output ofsaid sensor and said past corrections; and 5.c.ii) if said estimatedimbalance is above a second predetermined threshold, calculating acorresponding correction based on said estimated steady state rotationalimbalance.
 9. A laundry appliance as claimed in claim 7 or 8 whereinsaid correction means has a fine mode of control and a coarse mode ofcontrol and said step (5.d) comprises the following steps: 5.d.i) ifsaid estimated imbalance is above a second predetermined threshold butbelow a third predetermined threshold, then controlling said correctionmeans under said fine mode of control to add one or more masses to saiddrum, corresponding to said calculated correction; and 5.d.ii) if saidestimated imbalance is above a third predetermined threshold, thencontrolling said correction means under said coarse mode of control toadd one or more masses to said drum, corresponding to said calculatedcorrection.
 10. A laundry appliance as claimed in any one of claims 1 to6, wherein said sensor further comprises filtering means forconditioning the output of said sensor, said filtering means including:a low pass filter for filtering the output of said sensor and providingas its output a low passed output signal; position means for sensing theangle of said drum relative to a predetermined reference; and softwareprogrammed into said processor comprising the following steps: I)multiplying said low passed output signal at predetermined angles, ofrotation of said drum by a value according to the cosine of the angle ofsaid drum resulting, in a first product; ii) multiplying said low passedoutput signal at said predetermined angles of rotation of said drum by avalue according to the sine of the angle of said drum, resulting in asecond product; iii) adding the values of said first product at each ofa predetermined number of intervals over a full rotation of said drumand dividing the sum by the number of intervals thereof, to produce afirst result; iv) adding the values of said second product at each ofsaid predetermined number of intervals over a full rotation of said drumand dividing the sum by said number of intervals, to produce a secondresult; and v) supplying, a complex number composed of said first resultas the real component and said second result as the imaginary component,as the input to said processor in place of said output of said sensor.11. A laundry appliance as claimed in any one of claims 1 to 6 whereinsaid sensor comprises at least one piezo-electric force transducerprovided at each end of said drum adapted to detect the linear forcesacting on said drum resulting from the rotation thereof.
 12. A laundryappliance as claimed in claim 11 wherein said sensor further comprisesat least one acceleration transducer provided at each end of said drumadapted to detect the linear accelerations acting on said drum resultingfrom the rotation thereof.
 13. A laundry appliance as claimed in claim12 wherein said correction means comprise two sets of angularly spacedchambers in an orthogonal plane to the spin axis provided at each end ofsaid drum and means for injecting water into selected chambers inresponse to energisation by said processor.